This invention relates to a fluid pressure operated servomotor such as a vacuum booster or the like for use in a vehicle hydraulic braking system or the like.
A great variety of fluid pressure operated servomotors have been proposed and used in practice in vehicle hydraulic braking systems. One of prior art servomotors, such as shown in U.S. Pat. No. 3981227, comprises a main body which is divided into two air-tight chambers by means of a diaphragm-power piston unit connected to an output rod, an input rod for actuating a valve mechanism, and a reaction lever for transmitting the reaction force of a force generated in the output rod to the input rod. The reaction lever acts to transmit the reaction force simultaneously with the generation of the output force in the output rod, so that when the servomotor is used in the vehicle braking system, a reaction force will be transmitted to the input rod or to a brake pedal at the initial stage of the brake application when the output force is not sufficiently large to effectively apply the brake, which is not preferable for a good operational feeling of the brake pedal.
In order to eliminate the disadvantage referred to hereinabove, an improved servomotor has been proposed as shown in Japanese Utility Model Publication No. 53434/1976, in which a return spring urging the power piston unit in the return direction is utilized to act on the reaction lever through a retainer. The retainer is slidably mounted on a bolt threadedly secured to the power piston, and in the unactuated condition of the servomotor the force of the return spring is transmitted to the power piston through the retainer. Thus, there are problems that the construction is complicated, the assembling operation is not easy, and the amount of the reaction force will change according to the sliding resistance between the bolt and the retainer.